Polyacetal resins are well known engineering resins due to their balanced mechanical properties. In this regard, polyacetal resins usually exhibit desirable physical properties in terms of their frictional and abrasive characteristics, electrical properties and chemical and heat-resistance properties. Certain properties of the polyacetal resin are, however, required to be continually improved due to special environments of use.
The sliding characteristics of polyacetal resins are examples of properties which sometimes need improvement. In this regard, polyacetal resins have recently been employed as guide members for travelling magnetic tapes of audio and video machines, as well as for guide posts of tape cassettes. Other representative examples of sliding parts where strict requirements for sliding characteristics must be satisfied include guide members, such as guide rollers and guide posts for use in the tape travel system of VTRs, 8 mm VTRs and the like.
In general, guide members made of machined metallic materials have been employed in the above-noted end-use applications. When using metallic materials, however, it is necessary that precautions be observed in order to prevent damage to the tape due to sliding contact with the guide member during operation. In this regard, it is thus usually necessary to reduce the friction coefficient of metallic guide posts by subjecting the surface which contacts the tape to a surface-smoothing treatment, such as plating, polishing, lapping and the like. Such surface treatments are economically disadvantageous (i.e. the cost of the guide members increases) and necessarily require separately handling and/or special fabrication techniques in order to prevent the guide post from being damaged prior to use.
For these reasons, proposals have been made to incorporate various lubricants in resin guide members so as to provide alternative materials for conventional metals used in such end-use applications. Polyacetal resins having excellent mechanical properties, electrical properties, chemical resistance properties and the like have specifically been proposed to be employed as an alternative material to conventional metallic tape guide posts by incorporating various lubricants in polyacetal base resins. Examples of lubricants that have been proposed include fluororesins, polyolefin resins and silicone resins. In addition, solid lubricants, such as graphite and molybdenum disulfide, and lubricating oils such as paraffin oils, spindle oils and silicone oils, have been proposed. In particular, various proposals have beenadvanced regarding incorporating silicone oil (which exhibits excellent heat-resistance properties, low-temperature characteristics, and chemical and physical stability) into polyacetal base resins.
In general, most lubricants (including silicone oil) typically exhibit very poor compatibility characteristics with respect to their affinity to polyacetal resins. This incompatability between polyacetal resins on the one hand and lubricants (including silicone oil) on the other hand, thus makes it very difficult to incorporate such lubricants into the polyacetal resins in a homogenous manner during preparation.
Furthermore, even when compositions are successfully prepared, lubricating oils usually tend to bleed from the surface of the resin during molding so that relative sliding occurs between the individual resin particles and/or between the resin particles and the screw of a conventional screw extruder. This in turn causes the resin to experience poor "bite" by the screw and thus insufficient plasticization of the resin ensues. In extreme cases, such incompatability between lubricating oils and polyacetal base resin makes it impossible to conduct molding operations. As noted above, however, even if such molding is successfully performed, the lubricating oil usually "bleeds" from the surface of the molded article and thus transfers and/or adheres to the tape surface in contact therewith. As a result, the lubricant-contaminated tape usually is discolored. Furthermore, lubricating oils disadvantageously lower the surface hardness of the molded article.
As mentioned briefly above, several proposals have been made regarding improving the self-lubricity characteristics of polyacetal base resin including the addition of lubricating oils, particularly silicone oils. More specifically, it has been proposed in the past to introduce silicone oils into polyacetal base resins together with a carrier substance for the silicone oil so as to alleviate the above-described problems relating to dispersability and/or compatability of the silicone oil with respect to the polyacetal base resin. Examples of inorganic carriers that have been proposed in the past include active carbon and graphite, while examples of organic carriers include high molecular weight polyethylene, ethylene copolymers, and silicone rubber.
Active carbon and graphite, however, are incapable of satisfactorily absorbing and holding silicone oil. High molecular weight polyethylene and ethylene copolymers and the like similarly exhibit poor affinity to silicone oils and therefore are less than satisfactory. Silicone rubber, on the other hand, although exhibiting satisfactory compatability with silicone oil (and thus can considerably improve the processing and fabrication characteristics of the resin) is incapable of satisfactorily retaining the silicone oil under sliding conditions. Furthermore, silicone rubber flows when subjected to elevated temperatures thereby causing the rubber to congregate on the surface of molded resin articles. As a result, a thin film is formed on the surface of the molded article which easily peels off when subjected to external stress--i.e. as may be encountered by contact with a running audio/visual tape. Thus, the silicone oil carriers that have been proposed in the past do not alleviate the "bleeding" problems that have been encountered with respect to silicone oils.
Although the addition of fluororesins, olefin resins and the like serve to improve the sliding characteristics of polyacetal resin to some extent, such resins in and of themselves typically exhibit poor compatability with the polyacetal resin. Thus, surface-peeling of molded articles and/or formation of mold deposits during fabrication of such molded articles usually ensues. Furthermore, addition of such solid lubricants can cause the sliding properties to gradually degrade over time due to a powder film forming on the surface of the wear part due to the abrasion of the tape in contact with the surface of the molded article.
As is evident from the above discussion, the known prior art techniques for providing self-lubricity and/or anti-frictional characteristics to polyacetal resins have yet to produce satisfactory results for short-term and long-term sliding characteristics. Therefore, further improvements in this regard have been needed. It is towards satisfying such a need that the present invention is directed.
According to the present invention, improved self-lubricity properties may be obtained for polyacetal resin compositions by blending with a polyacetal base resin, a self-lubricating effective amount of silica powder in which a silicone polymer (e.g. a silicone oil) has been absorbed. The silica powder (which is highly compatible with the polyacetal base resin) is easily dispersed throughout the polyacetal base resin so that the lubricating properties of the silicone oil can be fully realized. Moreover, the silica powder satisfactorily retains the silicone oil in the composition. As a result, problems associated with "bleeding" of the silicone oil onto surfaces of molded parts formed of the compositions of the present invention are alleviated.
Preferably, the compositions of the present invention include a polyacetal base resin and, melt-blended therewith, between 0.5 to 25% by weight (based on the total composition weight) of a silica powder in which a silicone polymer has been absorbed. Molded articles, such as guide members or magnetic tape travel systems (particularly guide posts for tape cassettes) will thus exhibit satisfactory self-lubricity properties. The compositions of the present invention most preferably include between 75 to 99.5 wt % of a polyacetal base resin, and between 0.5 to 25 wt % of a silica powder in which between 20 to 80 wt % (based on the weight of the silica powder) of a silicone polymer is absorbed.
Further aspects and advantages of the present invention will become more clear after careful consideration.:s given to the following detailed description of the preferred exemplary embodiments thereof which follow.